1. Field of the Invention
The present invention relates to a transmission apparatus using a power amplifier (hereinafter “PA”).
2. Description of the Related Art
FIG. 1 shows an example of a related art transmission apparatus using polar modulation scheme. The transmission apparatus is provided with a polar signal generation circuit 1, an amplitude control circuit 2, a phase modulated signal generation circuit 3 and a PA 4. In this transmission apparatus, the polar signal generation circuit 1 generates a signal in accordance with the amplitude and phase of a transmit modulated signal from an input signal, and, based on this signal, the amplitude control circuit 2 and the phase modulated signal generation circuit 3 generate the power supply to PA 4 and a phase modulated signal, respectively.
In practice, the transmission apparatus maintains the dynamic range of transmission power by switching the PA 4 between compressed mode and noncompressed mode. That is, this transmission apparatus operates the PA 4 in compressed mode when high transmission power is required. On the other hand, the transmission apparatus operates the PA 4 in noncompressed mode when only low transmission power is required. To be more specific, in compressed mode, the transmission apparatus carries out amplitude modulation by changing the power supply to the PA 4 according to the amplitude signal. This mode is inherently very accurate. On the other hand, in noncompressed mode, the transmission apparatus operates the PA 4 in a less accurate compressed mode.
However, with related art transmission apparatus, there is a threat that, when compressed mode (“c-mode”) and noncompressed mode (“n-mode”) switch in transmission power control, drift of maximum 5 dB or more occurs in transmission power due to differences in characteristics between the modes (drift due to temperature, drift due to wear, drift due to load, etc.).
This will be explained briefly using FIG. 2. As shown in FIG. 2, output power of compressed mode is relatively accurate. On the other hand, output power of noncompressed mode changes due to drift due to temperature, drift due to wear, drift due to load, etc.
As shown in FIG. 2, output power of noncompressed mode tends to drift due to various factors and therefore is likely to be discontinuous when compressed mode and noncompressed mode switch. As a result, large drift in transmission power is likely to occur.
Now, according to 3GPP 25.101, the requirements shown in FIG. 3 to FIG. 5 need to be fulfilled in terms of transmission power errors. With related art transmission apparatus, however, it is difficult to fulfill these requirements provided in 3GPP 25.101, when compressed mode and noncompressed mode switch.